Elastic/plastic/cracking indentation behavior of hard materials

A Hertzian-based elastic/plastic hardness stress–strain analysis is employed to demonstrate that, for low temperature indentation measurements made on hard single crystals and polycrystals, the elastic and plastic indentation strains are of comparable magnitude, even for diamond pyramid microindentations or Berkovich trigonal nanoindentations. Examples are provided from reported test results determined for both types of indentations made on magnesia (0 0 1) crystal surfaces and for nanoindentations made on a sapphire crystal. New results are reported for a two-phase alumina/titanium diboride material system. One effect of neglecting the material elastic strain, particularly, in tests involving indentation parameters determined from small penetration depths, is to introduce an indentation size effect (ISE). Also, a depth measurement reported for a downwardly displaced “trough” spreading from an MgO nanoindentation is correlated with the indentation depth, in support of the primary indentation-forming dislocation activity. Such unique dislocation association applies as well for the secondary type {1 1 0} slip-induced cracking that occurs during indentation of the magnesia crystals, also, at lower hardness pressures than the theoretical elastic pressures required for cracking. Similarly, lower plastic hardness pressures are obtained for the new results on a two-phase alumina/titanium diboride material system.